Although long talked about, the Ubuntu Edge campaign exemplified the concept best with its "super phone" boast: your phone would hook up to a monitor, mouse and keyboard and become a fully functioning Ubuntu desktop PC. Phone apps would run on the desktop in an appropriate guise like responsive websites do on phones.

My dream smartphone would be a phone that automatically turns into a PC the moment I get home. It knows I'm home, wirelessly and automatically hooks up to my display, mouse, and keyboard in my office, and done. Of course, it'd also automatically detect other displays and input devices in my house - say, a remote control and my TV.

Mir is very much a dead-end after Canonical secretely forked Wayland and spread fear, doubt and uncertainty. They should drop that Not Invented Here software syndrome and improve the support to Wayland.
Considering the size of company, Canonical cannot afford to overstretch.